Embodiments of the present invention are directed generally to a method and an apparatus for converting a DC voltage to an AC voltage. More specifically, embodiments of the present invention are directed to methods and apparatus for converting DC voltages to AC voltages using resonant bridge inverter circuits in devices such as uninterruptible power supplies.
The use of uninterruptible power supplies (UPSs) having battery back-up systems to provide regulated, uninterrupted power for sensitive and/or critical loads, such as computer systems, and other data processing systems is well known. FIG. 1 shows a typical prior art UPS 10 used to provide regulated uninterrupted power. The UPS 10 includes an input filter/surge protector 12, a transfer switch 14, a controller 16, a battery 18, a battery charger 19, an inverter 20, and a DC-DC converter 23. The UPS also includes an input 24 for coupling to an AC power source and an outlet 26 for coupling to a load.
The UPS 10 operates as follows. The filter/surge protector 12 receives input AC power from the AC power source through the input 24, filters the input AC power and provides filtered AC power to the transfer switch and the battery charger. The transfer switch 14 receives the AC power from the filter/surge protector 12 and also receives AC power from the inverter 20. The controller 16 determines whether the AC power available from the filter/surge protector is within predetermined tolerances, and if so, controls the transfer switch to provide the AC power from the filter/surge protector to the outlet 26. If the AC power from the rectifier is not within the predetermined tolerances, which may occur because of xe2x80x9cbrown out,xe2x80x9d xe2x80x9chigh line,xe2x80x9d or xe2x80x9cblack outxe2x80x9d conditions, or due to power surges, then the controller controls the transfer switch to provide the AC power from the inverter 20. The DC-DC converter 23 is an optional component that converts the output of the battery to a voltage that is compatible with the inverter. Depending on the particular inverter and battery used the inverter may be operatively coupled to the battery either directly or through a DC-DC converter.
The inverter 20 of the prior art UPS 10 receives DC power from the DC-DC converter 23, converts the DC voltage to AC voltage, and regulates the AC voltage to predetermined specifications. The inverter 20 provides the regulated AC voltage to the transfer switch. Depending on the capacity of the battery and the power requirements of the load, the UPS 10 can provide power to the load during brief power source xe2x80x9cdropoutsxe2x80x9d or for extended power outages.
In typical medium power, low cost inverters, such as inverter 20 of UPS 10, the waveform of the AC voltage has a rectangular shape rather than a sinusoidal shape. A typical prior art inverter circuit 100 is shown in FIG. 2 coupled to a DC voltage source 18a and coupled to a typical load 126 comprising a load resistor 128 and a load capacitor 130. The DC voltage source 18a may be a battery, or may include a battery 18 coupled to a DC-DC converter 23 and a capacitor 25 as shown in FIG. 2A. Typical loads have a capacitive component due to the presence of an EMI filter in the load. The inverter circuit 100 includes four switches S1, S2, S3 and S4. Each of the switches is implemented using power MOSFET devices which consist of a transistor 106, 112, 118, 124 having an intrinsic diode 104, 110, 116, and 122. Each of the transistors 106, 112, 118 and 124 has a gate, respectively 107, 109, 111 and 113. As understood by those skilled in the art, each of the switches S1-S4 can be controlled using a control signal input to its gate. FIG. 3 provides timing waveforms for the switches to generate an output AC voltage waveform Vout (also shown in FIG. 3) across the capacitor 130 and the resistor 128.
A major drawback of the prior art inverter circuit 100 is that for loads having a capacitive component, a significant amount of power is dissipated as the load capacitance is charged and discharged during each half-cycle of the AC waveform. This power is absorbed by the switches S1, S2, S3, S4, which typically requires the switches to be mounted to relatively large heat sinks. The power dissipation issue becomes greater for high voltage systems, wherein the energy required to charge the load capacitance is greater. The dissipation of power in the switches dramatically reduces the efficiency of the inverter, and accordingly, reduces the run-time of the battery 18 in the UPS 10. Temperature rise of the switches also becomes a large concern.
In embodiments of the present invention, the problems associated with power dissipation in an inverter due to capacitor charging are eliminated by providing a resonant circuit in the inverter to charge and discharge the capacitor.
In one general aspect, the present invention features an uninterruptible power supply for providing AC power to a load. The uninterruptible power supply includes an input to receive AC power from an AC power source, an output that provides AC power, a DC voltage source that provides DC power, the DC voltage source having an energy storage device, and an inverter operatively coupled to the DC voltage source to receive DC power and to provide AC power. The inverter includes first and second output nodes to provide AC power to the load, first and second input nodes to receive DC power from the DC voltage source, a resonant element having a first terminal and a second terminal, the second terminal being electrically coupled to the first output node, a first switch electrically coupled between the first terminal of the resonant element and the first input node, and a set of switches operatively coupled between the first and second output nodes and the first and second input nodes and controlled to generate AC power from the DC power. The uninterruptible power supply further includes a transfer switch constructed and arranged to select either the AC line or inverter power source as an output power source for the uninterruptible power supply.
The set of switches in the inverter can include a second switch electrically coupled between the second output node and the second input node, a third switch electrically coupled between the second output node and the first input node, a fourth switch electrically coupled between the first output node and the first input node, and a fifth switch electrically coupled between the first output node and the second input node. The inverter can further include a sixth switch electrically coupled between the first terminal of the resonant element and the second input node.
The resonant element in the inverter may include an inductor. Each of the switches may include a transistor. The energy storage device can include a battery. The transfer switch can be constructed and arranged to receive the AC power from the input and to receive the AC power from the inverter and to provide one of the AC power from the input and the AC power from the inverter to the load.
In another general aspect, the present invention features an uninterruptible power supply for providing AC power to a load having a capacitive element. The uninterruptible power supply includes an input to receive AC power from an AC power source, an output that provides AC power, a voltage source that provides DC power, the voltage source having an energy storage device, an inverter operatively coupled to the voltage source to receive DC power and having an output to provide AC power. The inverter includes means for charging the capacitive element by supplying electrical current from the inverter to the load through a resonant element, means for supplying load current from the inverter after the capacitive element has been charged to a predetermined voltage, means for discharging the capacitive element through the resonant element, and means for transferring energy from the resonant element to the energy storage device. The uninterruptible power supply further includes a transfer switch constructed and arranged to select one of the AC power source and the voltage source as an output power source for the uninterruptible power supply.
In another general aspect, the present invention features a method of supplying an uninterruptible AC voltage to a load having a capacitive element using an uninterruptible power supply having a DC voltage source with an energy storage device. The method includes steps of charging the capacitive element by supplying electrical current from the DC voltage source to the load through a resonant element in the uninterruptible power supply, supplying load current from the DC voltage source to the load after the capacitive element has been charged to a predetermined voltage, discharging the capacitive element through the resonant element, and transferring energy from the resonant element to the energy storage device in the DC voltage source.
The method can further include steps of receiving an AC voltage from an AC power source, selecting one of the AC power source and the DC voltage source as an output power source for the uninterruptible power supply.